The Schottky diode is a power rectifier widely applied to many fields, such as power supply devices, power converters, motor controllers, telecommunication switches, factory automation apparatuses, electronic automation devices and low-voltage converters. The Schottky diode is indeed superior in switching speed. However, the Schottky diode still needs careful design in the materials and structures to achieve a low reverse leakage current, a high breakdown voltage and a low turn-on voltage so as to achieve the best performance.
In comparison with the silicon-based Schottky diode, the gallium nitride Schottky diode has a wider energy gap and a better carrier conduction performance. Therefore, the gallium nitride-based Schottky diode has a higher breakdown field and a lower on-state resistance. The gallium nitride-based Schottky diode further has a high switching speed and a low switching loss. Therefore, the gallium nitride-based Schottky diode plays an important role in application of rectifiers. Refer to FIG. 1. The conventional gallium nitride-based Schottky diode 1 comprises a substrate 2, a buffer layer 3, an N-type gallium nitride layer 4, an ohmic contact electrode 5, a Schottky electrode 6, and a protection layer 7, wherein a Schottky contact is formed between the Schottky electrode 6 and the N-type gallium nitride layer 4, and the buffer layer 3 makes the N-type gallium nitride layer 4 easily formed on the substrate 2.
The substrate of the gallium nitride-based Schottky diode is usually made of sapphire or silicon carbide. However, the poor heat conduction between sapphire and gallium nitride is unfavorable for use in high power application. Further, the silicon carbide substrate is expensive and has limited commercial application. Therefore, the silicon substrate, which is less expensive and has an appropriate thermal conductivity, has been gradually adopted recently.
Under a reverse bias, the electric field strength has a peak in the interface between the Schottky electrode and the N-type gallium nitride layer and gradually decreases from the interface toward the substrate and the ohmic contact electrode. When the peak strength is greater than the breakdown voltage of the N-type gallium nitride layer, or when the electric field strength in the substrate is greater than the breakdown voltage of the substrate, the Schottky diode breaks down.
The breakdown voltage of silicon (about 0.3 MV/cm) is lower than one tenth of the breakdown voltage of gallium nitride (about 3.3 MV/cm). Therefore, too thin an N-type gallium nitride layer is likely to cause the breakdown of the silicon substrate and results in a relatively lower breakdown voltage of the entire Schottky diode. When the gallium nitride layer has a thickness of 1-2 μm, the diode has a breakdown voltage of 100-300V. When the gallium nitride layer has a thickness of 5-6 μm, the diode has a breakdown voltage as high as 650V. However, a thicker gallium nitride layer is expensive. Further, the lattice mismatch between gallium nitride and silicon is very great and likely to generate crystal dislocation and result in poor reliability.